In the past twelve years, design of the high resolution color CRT electron gun has evolved from the individual type of main lens design to the common lens type design. In the individual type main lens design, inside each of the three guns (red, blue, green) the electron beam goes through an individually defined lens space without sharing this space with its neighbors. FIG. 1 is a simplified sectional view of a conventional individual type main lens 10 shown in terms of an optical analogy with three light beams. This type of design is simple and straightforward. However, the drawback of this design is that each gun has a very limited space, resulting in high spherical aberration and generally poor electron beam spot resolution at high beam current.
The so-called "common lens" design has a single, shared aperture for the three electron beams. Each of the three beams goes through its own individual beam path, plus a shared focusing region, as shown in the simplified sectional views of FIGS. 2a and 2b of a conventional common lens type main lens 12 such as used in a color CRT. The common lens design dramatically reduces spherical aberration in the horizontal direction (in the plane of FIG. 2a) and also somewhat reduces spherical aberration in the vertical direction (in the plane of FIG. 2b).
FIGS. 3a, 3b and 3c show various views of a so-called "solid common lens" 14 which has a shared lens focusing space. The rim 16 of the solid common lens 14 is defined by a doubly bent sheet of metal. This structure, called a solid common lens, makes the part mechanically stronger, but at the same time its opening, or the shared lens focusing space of the lens, is restricted by the double layered rim 16 which limits the extent of spherical aberration correction.
FIGS. 4a, 4b and 4c show various views of another type of common lens structure--the hollow common lens 18. In this type of design, the wall 20 of the common lens 18 is a single-layer sheet of metal. In a defined CRT neck size, a hollow common lens will have less lens spherical aberration compared to a solid common lens situated in the same neck size. However, the inward directed portion 20a of wall 20 also limits the extent of spherical aberration correction available in this type of common lens. Hollow common lens 18 is shown in FIG. 4b as including a conventional "body" portion 18a.
Another variable in the common lens design is the shape of the lens. As shown in the racetrack shape of FIG. 3a as well as the dogbone shape of FIG. 4a, in both the solid and hollow common lens designs the horizontal edges of the center gun are straight and parallel. This causes the center (green) gun to have a higher astigmatism than the two outer guns (red, blue).
The present invention addresses the aforementioned limitations of the prior art by providing a chain link-type common lens aperture in an electron gun main focus lens electrode having a thin peripheral side wall aligned parallel to the beam axes for reducing electron beam spherical aberration in a color CRT. The chain link-type common lens aperture may be employed in facing portions of adjacent electrodes in the main focus lens or in combination with either a facing solid or hollow common lens structure. This invention also contemplates facing hollow common lens structures in facing portions of adjacent electrodes in the electron gun's main focus lens. The chain link-type common lens aperture may be either disposed in an inwardly directed end wall of the electrode or it may be defined by a straight side wall of the electrode aligned with the electron gun axis.